SparkFun Forums

[roach]

Okay, this may seem obvious to anyone who's done electronics, but I figured it's better to ask a stupid question than to save face and fry my board.

I just purchased the atmega128 header board from Sparkfun (looks sweet!), and was thinking of hooking it up and starting to learn...

1) Can I hook this up to a 9-volt battery? The datasheet for the on-board regulator (LM78L05) says there's a minimum voltage of 6.8, so I am assuming this is alright. Am I wrong to assume this?

2) Someone (I think it was Kuroi) recently introduced me to the concept of "ground loops" (where the "ground" value in one area of the circuit is not the same as in another area, causing current where you don't expect it). Again, talking down my paranoia, can I plug the negative (minus) terminal of the 9V directly into the ground line on my board? Is this the same "ground" as the ground pins on the atmega128, or will I fry something?

3) Are the 9V battery terminals really +9V and GND? Or are they something else (like +4.5V and -4.5V or something).

Sorry for the dumb questions, but I'm still learning...

thanks!

[leon_heller]

[quote="roach"]Okay, this may seem obvious to anyone who's done electronics, but I figured it's better to ask a stupid question than to save face and fry my board.

I just purchased the atmega128 header board from Sparkfun (looks sweet!), and was thinking of hooking it up and starting to learn...

1) Can I hook this up to a 9-volt battery? The datasheet for the on-board regulator (LM78L05) says there's a minimum voltage of 6.8, so I am assuming this is alright. Am I wrong to assume this?

No problem with that. It will have an on-board regulator delivering 5 V to the AVR.


2) Someone (I think it was Kuroi) recently introduced me to the concept of "ground loops" (where the "ground" value in one area of the circuit is not the same as in another area, causing current where you don't expect it). Again, talking down my paranoia, can I plug the negative (minus) terminal of the 9V directly into the ground line on my board? Is this the same "ground" as the ground pins on the atmega128, or will I fry something?

You only need bother about ground loops with audio equipment, they can cause hum and instability. The power supply input ground should be effectively the same as the ground everywhere on the board, so you can ground scope probes and the like anywhere on the board. Just connect the battery positive to the positive supply input and the battery negative to ground via the connector. The board should be protected against incorrect supply polarity, I do that on most things I design.


3) Are the 9V battery terminals really +9V and GND? Or are they something else (like +4.5V and -4.5V or something).


They will probably be nearer 10 V and 0 V (ground) if it is a new battery. Won't matter, though.

Leon

[roach]

They will probably be nearer 10 V and 0 V (ground) if it is a new battery. Won't matter, though.

Thanks! I think this neatly answers all of my questions (unles by "won't matter" you mean "won't matter because you don't know what you're doing anyways" )

[Kuroi Kenjin]

With the ground loops... you really only have to worry about them in stuff that plugs into the wall. I had to worry about them when doing a co-op at Keithley testing multimeters and such. Ground loops also really screw up low level measurements.

With pure DC (batteries), whatever you connect together and call ground, is ground. The other voltages are plus or minus whatever magnitude.

With wall stuff, each equipment generates a "ground", which could be close to another equipment's "ground", but not quite since you're making DC voltages...etc. from an AC waveform.

A very fresh 9V is about 9.6V (6x 1.5V cells... max at 1.6V each fresh), although I've used really really dead 9V for powering circuit board for work in between classes at the Student Union... those 9V were measured around 7-8V... as long as they're about 6.5V (for 7805 regulator), or 5.5V (78L05)... they should work (and of course as long as you don't pull more much current at all... say 50-100mA).